Radioactive materials are often detected and identified by measuring gamma-rays and/or neutrons emitted from the materials. The energy of gamma-rays is specific to that particular material and acts as a “finger print” to identify the material. Similarly, neutron energy is particular to the material, and may be used to identify the material. Of very high value are detectors capable of identifying the distinctive time-correlated signatures corresponding to neutrons and gammas emitted by fissioning material from within a background of uncorrelated natural radiation. A detector capable of distinguishing neutrons from gammas, as well as offering a fast response time typically has better capability for detecting the distinctive time-correlated events indicative of the presence of fissioning nuclei.
The ability to detect gamma rays and/or neutrons is a vital tool for many areas of research. Gamma-ray/neutron detectors allow scientists to study celestial phenomena and diagnose medical diseases, and they have been used to determine the yield in an underground nuclear test. Today, these detectors are important tools for homeland security, helping the nation confront new security challenges. The nuclear non-proliferation mission requires detectors capable of identifying diversion of or smuggling of nuclear materials. Government agencies need detectors for scenarios in which a terrorist might use radioactive materials to fashion a destructive device targeted against civilians, structures, or national events. To better detect and prevent nuclear incidents, the Department of Energy (DOE) and the Department of Homeland Security (DHS) are funding projects to develop a suite of detection systems that can search for radioactive sources in different environments.
One particularly useful type of radiation detection, pulse shape discrimination (PSD), which is exhibited by some organic scintillators, involves subtle physical phenomena which give rise to the delayed luminescence characteristic of neutrons, providing a means of distinguishing neutrons from the preponderance of prompt luminescence arising from background gamma interactions. The mechanism by which this occurs begins with intersystem crossing (ISC), where the excited singlet state (S1) nonradiatively relaxes to the excited triplet (T), as shown in FIG. 1. In FIG. 1, the basic physical processes leading to the delayed fluorescence characteristic of neutron excitation of organics with phenyl groups is shown.
Since the triplet is known to be mobile in some compounds, the energy migrates until two triplets collide and experience an Auger upconversion process, shown as Equation 1:T1+T1→S0+S1  Equation 1
In Equation 1, T1 is a triplet, S0 is the ground state, and S1 is a first excited state. Finally, the delayed singlet emission occurs with a decay rate characteristic of the migration rate and concentration of the triplet population, which is represented as Equation 2:S1→S0+hv  Equation 2
In Equation 2, hv is fluorescence, while S0 is the ground state and S1 is a first excited state. The enhanced level of delayed emission for neutrons arises from the short range of the energetic protons produced from neutron collisions (thereby yielding a high concentration of triplets), compared to the longer range of the electrons from the gamma interactions. The resulting higher concentration of triplets from neutrons, compared to gamma interactions, leads to the functionality of PSD. The observation of PSD is believed to be in part related to the benzene ring structure, allowing for the migration of triplet energy.
It is generally accepted in the prior art that stilbene offers good PSD. However, stilbene, generally grown from melt, is difficult to obtain. Therefore, a number of other organic molecules have been examined. Unfortunately, most research in this area has concluded that other known liquid and solid materials, including many compounds having benzene rings, do not possess PSD properties comparable to single-crystal stilbene.
Accordingly, it would be beneficial to provide organic materials which may be comparable to or better than stilbene in relation to PSD properties for neutron radiation detection.